Flatten control flow pass (#999)

This pass flattens out control flow in order to achieve 2 properties:

 * Control flow structures (block, loop, if) and control flow operations (br, br_if, br_table, return, unreachable) may only be block children, a loop body, or an if-true or if-false. (I.e., they cannot be nested inside an i32.add, a drop, a call, an if-condition, etc.)

 * Disallow block, loop, and if return values, i.e., do not use control flow to pass around values.

As a result, expressions cannot contain control flow, and overall control flow is simpler, more structured, and more "flat".

This should make things like re-relooping wasm code much easier, as they can run after the cfg is flattened

Author: kripken

src/passes/CMakeLists.txt

@@ -5,6 +5,7 @@ SET(passes_SOURCES
   DeadCodeElimination.cpp
   DuplicateFunctionElimination.cpp
   ExtractFunction.cpp
+  FlattenControlFlow.cpp
   Inlining.cpp
   LegalizeJSInterface.cpp
   LocalCSE.cpp